Wavelength-converted laser systems often use nonlinear optical crystals, such as lithium borate (LBO) to generate a visible wavelength output beam from an infrared input beam generated by a source laser. In second harmonic generation (SHG), for example, a non-linear process taking place in the crystal combines two photons of infrared input radiation to produce a photon of visible output radiation having twice the frequency of the input infrared radiation. In third harmonic generation (THG), second harmonic generation is combined with an additional nonlinear optical crystal that is phase matched to combine a photon of the SHG output with a photon of the infrared input to produce third harmonic generation (THG) output having three times the frequency of the infrared input radiation.
Nonlinear crystals such as LBO are often characterized by an anisotropic refractive index, and this has an effect on the shape of the wavelength-converted output beam. An anisotropic refractive index means that the index of refraction depends on the direction of propagation and polarization of radiation in the crystal. If an unpolarized beam is launched into such a crystal, it will typically exhibit double refraction: the beam will split into two polarized beams that are not collinear but whose directions of propagation differ by an angle called the walk-off angle.
When a crystal with an anisotropic refractive index such as LBO is used for second harmonic generation, there will be walk-off among the beams. Either the output beam travels at a walk-off angle with respect to the input beam, or else the input beam itself undergoes double refraction, breaking up into two beams, one of which propagates at the walk-off angle. Because of the walk-off, the output beam is distorted and ends up with a different shape than the input beam. Typically the input beam is circular, and the output beam is elliptical.
A commercially viable third harmonic system would preferably have all or most of the following attributes. Such a system would be reliable and offer thousands of hours of hands-off operation. Output parameters, such as mode quality, beam pointing, and beam position would remain constant or nearly constant over long periods of time. The third harmonic beam produced by such a system would be round. The beam would have minimal asymmetry, e.g., less than about 15%. The beam would have minimal astigmatism, typically <20% of the Rayleigh length. The beam would approximate a diffraction limited Gaussian beam, typically M2<1.3. The output beam would be spectrally pure, that is residual fundamental and second harmonic light would be <1% of the third harmonic power. The system would convert fundamental radiation to third harmonic with an efficiency greater than about 30%, preferably greater than about 40%. The optical layout for third harmonic generation would also be easily optimized and contain relatively simple optical components. The system would flexibly operate over a wide operating range of pulse repetition frequencies and pulse widths.
It would also be desirable for the third harmonic output to be high power, with at least moderate energy pulses to efficiently process high volumes of material. For example, average power would be greater than 1 W, and preferably greater than 10 W with the potential to scale to higher power. The pulse energy would be greater than about 1 microjoule (μJ), preferably greater than 10 μJ with potential to scale to higher pulse energies.
Presently, there is no existing solution that would fulfill all these commercial requirements forcing customers make compromises and limiting the application of third harmonic systems.
Much of the prior art on third harmonic generation deals with intra-cavity systems. In an intra-cavity system, the third harmonic generation takes place within a resonator cavity defined by two reflective surfaces. An optical gain medium for the laser system is located within the cavity along with one or more non-linear optical crystals. The optical gain medium produces the fundamental radiation that undergoes frequency conversion in the non-linear optical crystals.
U.S. Pat. No. 5,850,407 to William Grossman discloses a type-I second harmonic generation (SHG) crystal followed by type-II third harmonic generation (THG) crystal within a laser cavity. This reference describes intracavity generation of third harmonic using a Lithium Borate (LBO) nonlinear crystal fabricated so that the output face is exactly at Brewster angle with respect to the fundamental and third harmonic beam. In this system, the output face must be at Brewster angle for the fundamental beam since otherwise the fundamental cavity would encounter additional losses. The Brewster surface provides three functions. First, it provides for wavelength separation of fundamental, second harmonic, and third harmonic beams from dispersion on the angled interface. Second, it provides for near-zero loss for IR and UV light. Third it is a high damage-threshold surface. Unfortunately, system in U.S. Pat. No. 5,850,407 requires an elliptical input beam and does not provide high single-pass conversion efficiency. Low single-pass conversion efficiency is not a problem in Q-switched intracavity harmonic generation since the fundamental beam is recirculated in the resonator cavity. Furthermore, since it only works efficiently intracavity, the system is not flexible since Q-switched lasers only operate over a narrow range of operating parameters.
It is noted that for a very large & round input fundamental beam, and in the low-efficiency limit, a Brewster-Brewster design (i.e., one in which both the input and output faces of non-linear crystal are Brewster angle faces) generates a round UV beam. This is essentially the operating point for the system described in the U.S. Pat. No. 5,850,407 patent. While a Brewster-Brewster design is useful in intra-cavity tripled systems, the single pass conversion efficiency is too low for extra-cavity harmonic generation.
Another example is described in U.S. Pat. No. 7,016,389. This patent discusses fabricating a wedge on the exit surface of the third harmonic crystal, which is generally smaller than the Brewster angle. Again this design is for an intracavity harmonic generation system. Extra-cavity designs require tighter focusing of the input beam in the nonlinear crystals than intracavity designs for efficient harmonic generation.
U.S. Pat. No. 7,170,911 describes an extra-cavity third harmonic nonlinear crystal fabricated with a Brewster angled output face. Alternatively such a system may be fabricated with coatings suitable to reduce reflections at normal or near-normal incidence output face. However, it is undesirable to put such coatings on a surface that receives a significant flux of ultraviolet radiation.
It is within this context that embodiments of the present invention arise.